During compressor shutdown, refriqerant accumulation and absorption takes place in the oil sump or crankcase and thereby dilutes the lubricating oil resulting in a refrigerant and oil mixture. The refrigerant accumulates in the compressor because it is at the lowest point in the system, due to the thermal gradient in the system and because of the affinity of halocarbon refrigerants for oil. Under normal operating conditions, some oil circulates with the refrigerant and will be returned to the compressor sump during continuous operation. In the case of a low side oil sump, there is a violent foaming that takes place upon start up due to the reduction of pressure and this produces a high oil circulation rate at this time.
Low concentrations of refrigerant in the compressor oil at start up is essential for long compressor and motor life, and satisfactory operation. The compressor is isolated from the system at shutdown through the compressor discharge valve at the outlet of the cylinder and a solenoid valve in the liquid line. Refrigerant is pumped out of the low side of the system at shutdown. A single pump out by closing the liquid line solenoid valve at shutdown may be used or the pump-down may be repeated automatically during shutdown as low side pressure rises. Repeated or continuous pump-down can cause a significant pumping of oil which is not returning to the compressor because of the short pumping cycle. To prevent the pumping out of all the oil, an oil safety switch is often employed to disable the compressor if there is an insufficient amount of oil. The use of an oil safety switch does not provide a complete solution since it must be bypassed on start up and when the system changes pressures.
Also, they are unreliable in the sense that they are subject to nuisance shutdowns, and expensive.
There are a number of situations where compressor operation will take place as a series of short cycles with the potential for causing the pumping out of the oil from the compressor. First, where there is a system refrigerant leak and a partial loss of the refrigerant charge there will be a repeated opening of the low pressure switch with a restart or reset since the thermostat will remain unsatisfied. Second, where the system is idle for an extended time but there is a periodic pump-down to keep the compressor dry. Third, where there is a valve leak and the compressor rapid cycles to keep the compressor dry.